If you were asked what you learned from a list of people you could recall to any extent, could you give an answer for each of them? It doesn’t have to be something new, good, or useful that you learned from them, necessarily, just something. Can you?
The quote is learn something from everyone you meet, without criteria on what you learned besides you could recall enough of something about each person to give an answer. That is why I can plausibly claim I really do learn something from everyone I meet, and not just preach it. I really do ask myself what I learned about someone new I met soon after I meet them, albeit not always consciously as if I were having a conversation with myself in my mind. It’s often just a quick memory check of my listening skills to recall some things about them, and identify some of it to be what I could learn from them to be a better person. That could range from a common behavior, like how they smiled a lot or were curious to ask stuff or interrupted me a lot, to things I hadn’t known, observed, or experienced before. The fact I’m not always learning something new, good, and/or useful each time doesn’t make the practice any less effective. Reinforcements are useful, too!
Listening is not a natural skill for me, though I can be good at it when I devote myself to it. I just have to devote myself to it to be in the moment of conversations more often. That’s the unnatural part for me. Additionally, repetition is good reinforcement. Finally, the mindset I get from repeatedly confirming I can learn something from everyone I meet not only helps me keep my humility, but also my curiosity and my interest in people.